1. Field of the Invention
The present invention relates to an engine.
2. Description of the Related Art
In recent years, the combustion pressure of engine has become higher and higher to achieve a high output power. Therefore, it is desired to provide a cylinder block having high rigidity and high strength to withstand the higher combustion pressure. As a result, in such an engine, since attack to a piston ring which slides against the cylinder is increased, it is necessary to improve durability (wear resistance, etc.) of the piston ring. There is conventionally known a piston ring having a nitride chromium film formed on an outer circumferential sliding portion thereof (refer to, for example, Patent Document 1: Japanese Patent Laid-Open Publication No. Hei07-286262, paragraphs [0023] to [0026] and others). Such a piston ring is excellent in durability compared with a conventional piston ring plated with hard chromium or a conventional piston ring subjected to a nitriding treatment.
Further, there is conventionally known a piston ring coated with a VN-based film. This type of the piston ring is more excellent in durability, and particularly excellent in crack resistance and peeling resistance (refer to, for example, Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-256967, paragraphs [0004] to [0007] and others).
On the other hand, in order to reduce the weight and size of the engine, a cylinder block formed of an aluminum-based composite is employed (refer to, for example, Patent Document 3: Japanese Patent Laid-Open Publication No. 2005-186151, paragraphs [0010] to [0014] and others). The aluminum-based composite is formed by, for example, incorporating single filaments or particles formed of alumina (Al2O3), silica (SiO2), carbon or the like into a matrix metal such as ADC12, so that the weight of the cylinder block can be reduced. However, the rigidity and strength of the aluminum-based composite is not sufficient compared with a cast iron, therefore combustion pressure of the engine can not be sufficiently increased.
Further, there has been known an aluminum-based composite formed by filling aluminum into pores of a porous ceramic, and Patent Document 4 (Japanese Patent Laid-Open Publication No. 2006-2606, paragraphs [0021] to [0031] and others) discloses a cylinder block whose cylinder liner portion is formed of the aluminum-based composite. Since the pores of the porous ceramic has a three-dimensional network structure formed by a plurality of spherical cells and communication holes which communicate the plurality of spherical cells adjacent to each other, the cylinder block is imparted with a rigidity and strength high enough to withstand a higher combustion pressure.
However, on the other hand, attack to the piston ring is further increased for the cylinder block formed by the porous ceramic (refer to, for example, Patent document 4) as compared with the cylinder block formed by the aluminum-based composite (refer to, for example, Patent Document 3). Thus, there is a concern that even if a piston ring which exhibits excellent durability against a conventional cast iron cylinder block (refer to, for example, Patent Document 1 and Patent Document 2) will not have sufficient durability against the cylinder block disclosed in Patent Document 4. Thus, it is necessary to provide a piston ring with high durability for a piston to be paired with the cylinder block formed by the porous ceramic.